Corrosion of artistic and historic metalwork is the primary means for their deterioration, while developing methods to better protect these works and monitor the efficacy of protective treatments remain central research questions (Degrigny, J. Solid State Electrochemistry 14, 3:353-361, 2010; Cano, et al., J. Solid State Electrochemistry 14, 3:381-391, 2010; Clare, et al., Proceed. of the ICOM-CC Metal 07 WG, Protection of Metal Artifacts, 83-87, 2007). To prevent degradation and corrosion of outdoor artwork, pigmented or clear protective coatings must be applied periodically because of the effects of pollutants, salts, light and moisture. Monitoring methods must be non-invasive so as to not damage or alter the substrate, and they must measure a relevant parameter. Coating gloss, thickness and/or wettability are commonly reported parameters used to monitor changes induced by weathering over time. Collections care personnel rely on visible changes (e.g. chalking, flaking and corrosion) to determine when coating reapplication is needed. However, these changes are cues of coating failure and signal that non-reversible damage to the underlying artwork has already occurred. While changes in the visual appearance of coatings are important, they may not directly correlate with changes in the permeability or porosity of a coating. It is conceivable that a coating may thin and lose gloss while remaining protective or become porous to electrolytes while maintaining film thickness.
Both electrochemical measurements and corrosion processes rely on the movement of electrons; thus electrochemical monitoring is perfectly suited to detect corrosion. There is a need to monitor the performance of coatings in situ to 1) verify that freshly applied coatings have acceptable barrier properties and are continuous, and 2) to ensure that weathered coatings have remained protective.
Electrochemical Impedance Spectroscopy (EIS) can be used to detect changes in the barrier properties of weathering coatings because it is fast, non-destructive, and being a frequency-based measurement, gives insight into the time constants of various surface-layer processes. Despite these advantages, three significant hurdles have prevented widespread use of EIS by the conservation community: 1) electrochemical testing typically requires artwork to be conductive and wired directly to instrumentation at an uncoated region of the artwork; 2) field-deployable equipment for these measurements is not available; and 3) rigid fluid-filled chambers are incompatible with the contoured surfaces of most outdoor metalwork.